Intermittent JRuby json issue related to tiered or G1

[Charles Oliver Nutter]

Hello again folks, we seem to have stumbled on an issue with either
tiered compilation or G1 or both.

https://github.com/jruby/jruby/issues/6554

We have been trying to track down a sporadic parse error in JRuby's
json library, and it now seems likely that it is an issue at the JVM
level. The issue above describes the problem that at least two of our
users have been seeing. The issue manifests as JRuby's json library
suddenly failing to parse code that worked earlier in the run.

The main user contributing details here, "Freaky", has been able to
confirm that either turning off tiered compilation or switching away
from G1 (to Parallel or Shenandoah) makes the problem go away, and has
provided a small reproduction:

https://github.com/Freaky/jruby-issue-6554

Freaky has also been trying to correlate JIT and GC logs to the time
of failure and has posted his findings on the JRuby issue above.
Freaky has been running the latest Java 15 to reproduce, but other
users are seeing the same issue on 11.

I wanted to get some feedback here before opening an issue, mostly
because it seems to require both tiered and G1 to trigger. Anyone have
some cycles to help us investigate?

- Charlie